Field of the Invention
The present invention relates to VCSELs (vertical cavity surface emitting lasers) that can be used for optical transmission links with high data rates and that have a photodetector that is monolithically integrated in the resonator between the resonator end mirrors (Distributer Bragg Reflector (DBR) gratings).
The electrical and optical properties of laser diodes, for example, the threshold current and the differential efficiency, vary from component to component. The properties of each component depend on the temperature, and they are subject to long- and short-term fluctuations. For this reason, it is necessary to provide an electrical feedback signal, which gives direct information about the actual optical output power of the laser and which can be used to regulate both the DC bias voltage and the modulation depth of the laser current during transmission. The costs associated with constructing a device for injecting an admittedly small, but nevertheless non-negligible part of the laser radiation into an external photodetector have led to the development of monolithically integrated components.
A range of structures with monolithically integrated photodetectors in the Bragg gratings that are used as a reflector are described in the publications by T. Kim et al.: xe2x80x9cA Single Transverse Mode Operation of Top Surface Emitting Laser Diode with an Integrated photo-diodexe2x80x9d in Proc. LEOS 1995, pp. 416-417, October 1995; by S. F. Lim et al.: xe2x80x9cIntracavity Quantum-Well Photodetection of a Vertical-Cavity Surface-Emitting Laserxe2x80x9d in Proc. Int. S. C.-Laser Conf. October 1996, Haifa/Israel, pp. 183-184; and by J. A. Lott et al.: xe2x80x9cDeep Red Vertical Cavity Surface Emitting Lasers With Monolithically Integrated Heterojunction Phototransistors For Output Power Controlxe2x80x9d in Proc. Int. S. C.-Laser Conf. October 1996, Haifa/Israel, pp. 185-186. Further sources are U.S. Pat. Nos. 5,757,837, 5,742,630, 5,577,064, 5,606,572 and 5,136,603.
The published laser structures with integrated photodetectors do not satisfy all of the following minimum conditions: easy production without losing yield on the wafer; good contrast in relation to spontaneous emission and ambient light, so that both the laser threshold and the differential efficiency can be established; few or no additional requirements in terms of the voltage that will be applied for the overall system; reproducible and frequency-independent feedback properties and only minor alteration of the optical and electrical properties over long operating times (deterioration).
Photodetectors in the semiconductor resonator are virtually insensitive to ambient light or scattered light. Since the detector properties of these photodetectors are predominantly based on the properties of the epitaxial layer growth, a good yield of functional components can be achieved even when fabricating with large tolerances. In order to produce components whose photodetectors provide a good contrast in relation to spontaneous emission, either the active detector region may be reduced to the size of the laser spot, although this raises additional technical problems and therefore reduces the yield (I. Y. Han, Y. H. Lee: xe2x80x9cOxide-apertured photo-detector integrated on VCSELxe2x80x9d in Proc. CLEO ""99, p. 176), or the sensitivity in relation to the coherent light in comparison with the spontaneous emission may be increased by a thin absorbing region at an antinode of the standing wave. By reducing the distance of this thin absorbing region from the active region, the contrast in relation to spontaneous emission can be increased further. However, this also increases the absorption of the laser radiation, so that the differential efficiency is reduced, which leads to very high sensitivities of the detector of about 1 A/W.
Another problem, in an integrated component, is avoiding higher laser impedances in comparison with an individual VCSEL, and therefore avoiding difficulties when using a driver circuit for the radiofrequency modulation of the laser current. Furthermore, electrical crosstalk between the laser and the detector must be minimized if unmodified RF components of the monitor signal are needed, for example, to adjust the DC bias voltage and the modulation depth.
It is accordingly an object of the invention to provide a VCSEL which overcomes the above-mentioned disadvantages of the prior art apparatus of this general type.
In particular, it is an object of the invention to provide an improved VCSEL with a monolithically integrated photodetector, that is straightforward to produce and that has a sufficiently low laser impedance, little crosstalk and good contrast in relation to spontaneous emission.
With the foregoing and other objects in view there is provided, in accordance with the invention, an optoelectronic component serving as a VCSEL, including: DBR gratings serving as reflectors; an active layer intended for generating radiation between the DBR gratings; a photodetector provided with a radiation-absorbing layer inside one of the DBR gratings, the radiation-absorbing layer being configured to overlap an antinode of a laser mode of the radiation that is generated; and a thick and heavily doped spacer layer located between the active layer and the radiation-absorbing layer.
In accordance with an added feature of the invention, the laser mode has a photon energy; and the radiation-absorbing layer has an energy band edge that is slightly lower than the photon energy of the laser mode.
In accordance with an additional feature of the invention, the photodetector and the spacer layer have a nonresonant optical thickness with respect to the laser mode so that an electric field distribution of the radiation inside the DBR gratings remains unmodified.
In accordance with another feature of the invention, the laser mode has a photon energy; and the spacer layer has an energy band edge that is slightly higher than the photon energy of the laser mode.
In accordance with a further feature of the invention, at least one semiconductor layer adjoins the radiation-absorbing layer and is heavily doped to be n-type conductive; and the spacer layer is heavily doped to be n-type conductive.
In accordance with a further added feature of the invention, there is provided, a region adjacent the radiation-absorbing layer; and at least one semiconductor layer adjoining the radiation-absorbing layer and having a grading in an energy band gap so that in the region adjacent the radiation-absorbing layer, the energy band gap grows toward the spacer layer.
In accordance with a further additional feature of the invention, there is provided, a depletion layer; and a device for applying an electrical voltage to modify the absorption in the radiation-absorbing layer. The radiation-absorbing layer lies inside the depletion layer.
In accordance with yet an added feature of the invention, there is provided, a depletion layer having an n-doped edge region; and a structural device for applying an electrical voltage to modify the absorption in the radiation-absorbing layer. The radiation-absorbing layer lies close to the n-doped edge region of the depletion layer.
In accordance with yet an additional feature of the invention, the photodetector is a pin photodiode.
In accordance with yet another feature of the invention, the photodetector is a bipolar phototransistor or a heterobipolar phototransistor.
In accordance with yet a further feature of the invention, the photodetector includes two photodiodes that have a common anode.
In accordance with an added feature of the invention, the photodetector includes two photodiodes that have a common cathode.
In accordance with an additional feature of the invention, the photodetector includes two photodiodes that have a common tunnel contact.
The component is a VCSEL with a photodetector, which is integrated in one of the DBR gratings that are provided as resonator mirrors, so that no mechanical adjustment of an external monitor diode is necessary. An active region, which is intended for radiation generation, is situated between the DBR reflectors. The coherent radiation generated in the resonator is emitted at the surface of the component. The photodetector in one of the DBR reflectors includes a thin absorbing layer, which is arranged in the vicinity of an antinode of the standing wave of a laser mode. The energy band edge of the material in the thin absorbing region is selected to be slightly lower than the energy that corresponds to the frequency of the emitted radiation, in order to prevent an irreproducible response of the detector, but without absorbing the low-energy part of the spontaneous emission.
The laser and the photodetector are electrically driven by a common electrode, preferably an n-type contact. This contact is situated on a spacer layer, which is doped with n-type conductivity at a dopant concentration that is sufficient to guarantee a low ohmic impedance inside the layer, and also to ensure a good ohmic metal-semiconductor contact. This spacer layer ensures a low laser impedance and little electrical crosstalk between the laser and the photodetector.
This spacer layer, or a further layer between the laser-active region and the photodetector, is preferably selected in such a way that the coherent light passes through this layer, but the high-energy part of the spontaneous emission is absorbed, so that a lowpass filter is thereby formed for the spontaneous emission. Together with the property of the thin absorbing region that it form a highpass filter, a bandpass filter is hence formed overall. The transmission range of the bandpass filter lies around the laser radiation frequency and hence further increases the contrast in relation to spontaneous emission.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a VCSEL With Monolithically Integrated Photodetector, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.